(a) Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery and a method of preparing the same and, more particularly, to a negative active material for a rechargeable lithium battery which exhibits high reversible capacity and low irreversible capacity.
(b) Description of the Related Art
Rechargeable lithium batteries such as lithium ion batteries, and lithium ion polymer batteries employ carbonaceous materials for the negative active materials. The carbonaceous materials can be largely classified into two categories of crystalline graphite and amorphous carbon. Crystalline graphite includes nature graphite and artificial graphite, the artificial graphite being obtained by sintering pitch at 2000° C. Amorphous carbon exhibits low degree of graphitization and displays very weak peaks in X-ray diffraction. Amorphous carbon includes soft carbon obtained by sintering coal pitch or petroleum pitch, and hard carbon obtained by sintering a polymer resin such as phenol resin.
Crystalline graphite exhibits good voltage flatness and high charge/discharge efficiency. With regard to amorphous carbon, although this material exhibits a high discharge capacity, it has a high irreversible capacity, low charge/discharge efficiency, and bad voltage flatness. Therefore, crystalline graphite is generally used for the negative active material in the rechargeable lithium battery.
When this active material (i.e., crystalline graphite) is coated on an electrode plate and pressed, the irregularly-shaped active material become fully aligned. Thus, a basal plane of the active material, to which lithium ions are not easily intercalated and deintercalated, makes contact with an electrolyte. Furthermore, since a graphene sheet develops in edge portions of the crystalline graphite, side reactions with an electrolyte become more severe. Thus, it is difficult to use crystalline graphite in rechargeable lithium batteries which require high initial charge and discharge efficiency (Journal of Electrochemical Society 137 (1990) 2009). In particular, if an electrolyte including propylene carbonate is used in the lithium secondary battery utilizing crystalline graphite as the negative active material, the crystalline graphite layer is separated from the electrode because of the co-intercalation of the electrolyte. As a result, the lithium ions do not intercalated and deintercalated in a normal fashion such that the initial efficiency of the active material and the capacity of the battery are reduced.
To address such problems, there have been attempts to produce carbonaceous material made out of a mixture of both crystalline carbon and amorphous carbon, thereby obtaining the advantages of both these materials.
Japanese Patent Laid-open No. Hei 8-180903 discloses a method of coating amorphous carbon on crystalline graphite. Japanese Patent Laid-open No. Hei 6-36760 discloses a method in which graphite particles are physically mixed with amorphous carbon fiber. Japanese Patent Laid-open No. Hei 6-275270 discloses a method in which crystalline graphite is physically mixed with amorphous carbon and the mixture is coated on a phenol resin. Japanese Patent Laid-open No. Hei 6-84516 discloses a method of coating graphite with amorphous cokes. Japanese Patent Laid-open No. Hei 5-325948 discloses a method of producing a composite of crystalline graphite and amorphous resin. In the methods, the amorphous resin is produced by cross-linking amorphous carbon.
Although the active materials obtained by the above methods exhibit the advantages of both crystalline graphite and amorphous carbon, the disadvantages of these two materials also appear in the resulting active material.
There have been attempted to decrease irreversible capacity by adding a catalyst to the active material. The methods using boron-based compounds as the catalyst are disclosed in Japanese Patent Laid-open No. Hei 8-31422, Hei 9-63584, Hei 9-63585, Hei 8-306359 and Hei 8-31422. In these methods, carbonaceous material is mixed with boron-based compounds and the mixture is heat-treated.
However, since the heating process is performed at 2000° C. higher in these methods, the methods are not economical. Although some examples in the above Japanese patent utilize a process in which natural graphite is wetted with a boron aqueous solution and carbonized at 1000° C. to obtain an improvement in electrical properties, graphite does not react well with boron at 1000° C. That is, these examples and their attendant advantages are based on an assumption of a direct reaction between boron and crystalline graphite. However, it is well known that boron-based compounds react with graphite at 2100° C., thus it is difficult to obtain the desired effect based on the direct reaction between boron and crystalline graphite.